Strange problem with a Fender Blues Junior - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 15th February 2012, 02:08 PM   #1
Cassiel is offline Cassiel  Libya
diyAudio Member
 
Cassiel's Avatar
 
Join Date: Sep 2004
Location: Madrid
Default Strange problem with a Fender Blues Junior

This is a push pull amp with a pair of EL84s. One tube has 70 volts 'missing' from the plate. I have changed the tube and it makes no difference. Bias voltage and screen voltage are normal but.....where do those 70 volts get lost?

320 volts depart B+ on their way to the plates. One plate gets 317V, the other one only gets 250V. Schematic says there's only two parts between B+ and the tube's plate. The OT and a diode to ground. The diode seems fine, the OT measures 190 Ohms across the primary winding, 95 Ohms between B+ and each plate. Looks OK too. I have no idea what's going on here, with no tubes both plate pins show the same readings. The tube is not the problem.

http://www.blueguitar.org/new/schem/fender/blues_jr.gif
__________________
Do your best and don't worry.
  Reply With Quote
Old 15th February 2012, 02:17 PM   #2
diyAudio Member
 
Osvaldo de Banfield's Avatar
 
Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
Could you see if the amplifier is oscillating? In this case, a VOM can give a false reading.

Good luck.
__________________
LW1DSE
  Reply With Quote
Old 15th February 2012, 02:32 PM   #3
Cassiel is offline Cassiel  Libya
diyAudio Member
 
Cassiel's Avatar
 
Join Date: Sep 2004
Location: Madrid
Thanks. I don't have the gear to check that out but when I attach the VOM probes to the circuit this strange noise comes up, it could be oscillating....but why everything reads normal except at the plate of that tube? Could be the OT?
It would be odd but maybe with current through the windings the OT starts misbehaving. A wild guess, I know.
__________________
Do your best and don't worry.
  Reply With Quote
Old 15th February 2012, 02:35 PM   #4
diyAudio Member
 
firechief's Avatar
 
Join Date: Sep 2008
Location: Near Seattle Wa
With no signal applied, measure the voltage on pin 2 of each EL84 they should both be aprox -10 V dc. If not check the values of R20 through R23. One tube is drawing more current than the other. You could also reverse the position of the two outputs and see if the low voltage follows the tube. Frankly it sounds like a bad tube. Good luck.
__________________
"You can keep your insurance baby nothing is guaranteed" -Tom Petty
  Reply With Quote
Old 15th February 2012, 02:39 PM   #5
diyAudio Member
 
Osvaldo de Banfield's Avatar
 
Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
Try removing the previous tube stage and re-read the voltages. This way, the amplifier must be stop oscillating. The reason you see only one plate at abnormal voltage depend of the waveform of the oscillation and its amplitude.
__________________
LW1DSE
  Reply With Quote
Old 15th February 2012, 02:51 PM   #6
Cassiel is offline Cassiel  Libya
diyAudio Member
 
Cassiel's Avatar
 
Join Date: Sep 2004
Location: Madrid
Quote:
Try removing the previous tube stage and re-read the voltages.
That did it. Thank you very much, everything reads OK now.
__________________
Do your best and don't worry.
  Reply With Quote
Old 15th February 2012, 03:09 PM   #7
diyAudio Member
 
Osvaldo de Banfield's Avatar
 
Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
OK, well go back into the circuit to find the fault.
__________________
LW1DSE
  Reply With Quote
Old 15th February 2012, 03:29 PM   #8
Cassiel is offline Cassiel  Libya
diyAudio Member
 
Cassiel's Avatar
 
Join Date: Sep 2004
Location: Madrid
I'm not going back. Amp plays fine and it's not mine. It only oscillates when my hand gets near the circuit. Not very stable. A friend plays a gig next week and he wants no surprises so he ask me to take a look and see if everything was OK with the amp. I did take a look and took some measurements and that's it. He's not getting any more work from me. Again, thanks for the help.
__________________
Do your best and don't worry.
  Reply With Quote
Old 15th February 2012, 03:51 PM   #9
diyAudio Member
 
Osvaldo de Banfield's Avatar
 
Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
OK.
__________________
LW1DSE
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
New Problem with Fender Frontman 212R diynow89 Instruments and Amps 19 22nd December 2013 07:33 PM
Preamp issue on Fender Blues Jr. Rotus623 Tubes / Valves 15 24th January 2011 04:13 PM
Sunfire True Sub Junior Problem sjdemark Subwoofers 2 15th July 2009 04:31 PM
Fender Blues Deluxe cab built HELP! chucknorris Swap Meet 0 7th September 2008 01:18 AM
blues junior complaints and 6G15 question layt Tubes / Valves 4 2nd July 2006 12:38 AM


New To Site? Need Help?

All times are GMT. The time now is 05:13 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2